Observer Pattern

Welcome to the lesson on the Observer pattern! This pattern is a crucial part of Behavioral Design Patterns, focusing on establishing a one-to-many relationship between objects. When one object changes state, all its dependents are notified and updated automatically. It's a common pattern used in scenarios where an object needs to broadcast changes to multiple other objects.

In this lesson, you will:

• Understand the basics of the Observer pattern.
• Learn how to implement this pattern in Java.
• Recognize the significance of the Observer pattern in real-world applications.

The Observer pattern consists of two key components: the Subject and the Observer. The Subject holds the state and notifies observers about any changes. The Observers receive updates whenever the Subject's state changes. This pattern is useful for broadcasting updates to multiple objects while maintaining decoupling since the Subject only knows the observers implement a specific interface.

In this lesson, we’ll go through a step-by-step example of implementing the Observer pattern. We’ll define a contract for the observers, create concrete implementations for both observers and the subject, and build a news system where subscribers receive updates about breaking news. This will help you grasp the practical application of the Observer pattern.

Let's break down the implementation step-by-step using our example.

public interface Subscriber {
    void update(String news);
}

The Subscriber interface defines a contract for all observers. Any class implementing this interface must provide an implementation for the update method, which receives the updated information.

public class NewsSubscriber implements Subscriber {
    private String name;

    public NewsSubscriber(String name) {
        this.name = name;
    }

    @Override
    public void update(String news) {
        System.out.println(name + " received news: " + news);
    }
}

Here, NewsSubscriber is a concrete implementation of the Subscriber interface. Each subscriber has a name and an update method that prints the received news.